1 INTRODUCTION

Among the structural components in monumental buildings, masonry arches and vaults deserve par-ticular attention. They are very widespread in Italian historical centers, and their preservation as part of the cultural heritage is a very topical subject.

Because of their ages or for accidental causes (such as earthquakes), these structures can suffer several types of damage, so the contribution of strengthening materials and repair techniques may be required to re-establish their performances and to prevent the brittle collapse of the masonry in possi-ble future hazardous conditions. According to this, strengthening masonry vaults poses serious concerns because the vast majority is of considerable architec-tural and historical value.

Traditional reinforcement techniques may guar-antee an adequate increment in strength, stiffness, and ductility, but are often short-lived and labor-intensive, and they usually violate aesthetic re-quirements or conservation or restoration needs. Re-cent earthquakes have demonstrated how such inter-ventions, based on reinforced concrete and steel rebar, appeared extremely harmful for structures be-longing to the architectural heritage. After recogniz-ing damages of those “seismic upgrading” following the regulations, members of National Committee for Cultural Heritage Seismic Risk Prevention claimed the principle of seismic improvement by techniques respecting the structural system and preserving their integrity. This way of thinking have been clearly

claimed by 10.29.1996 decree explaining the manda-tory seismic improvement for historically – artisti-cally relevant structures as interventions on the structural elements of the building in order to in-crease the safety margin without changing the main features of global behaviour. The current debate about restoration and consolidation of historical con-structions assumes, in fact, that an historical build-ing is the primary source of knowledge, a significant testimony in its full complexity. Thus, it is essential to deal with the individual object as a unique, unre-peatable instance, assigning equal value, dignity, importance, and right to protection to all the compo-nents of the building and all the material evidence contained in it. Hence, a strengthening project has to be preceded by a scientific diagnostic approach and has to minimise the impact of the intervention, by choosing the most compatible solution with respect to the building’s current state, with the aim of pre-serving it as better is possible. Therefore, the actual approach to restoration leads to the requirement of new reinforcement technologies, able to work in parallel and in cooperation with the existing struc-tures, and moreover characterized by the fact to be light, durable and possibly removable.

2 STRENGTHENING OF MASONRY ARCHES AND VAULTS

Thanks to their adaptability to the changes of the geometric configuration, masonry arches or vaults

Research on composite strengthening of historical housebuilding: retrofitting intervention for masonry arches and vaults

A. Borri & G. Castori Department of Civil and Environmental Engineering, University of Perugia, Perugia, Italy

ABSTRACT: A Seismic retrofitting of structures belonging to the architectural heritage requires meeting of constraints which are related to preservation of artistic features. Such a philosophy was applied to the design process of the retrofitting intervention of a masonry arch, belonging to a 17th century portico built inside a Roman amphitheatre in the city of Spoleto (Italy), and of two masonry vaults of an ancient building in the city of Foligno (Italy). This led to the opportunity of investigating the efficiency of an innovative composite mate-rial, based on fine steel cords embedded in a cementitious matrix (Steel Reinforced Grout). To assure an ade-quate strength against earthquake, a couple of prestressed SRG strips, in the first case, and a reticular system, made of transversal and longitudinal prestressed SRG laminates, in the second case, have been applied on the extrados of the arch and of the vaults, respectively.

are able to distribute the strain along the mortar joints, avoiding the formation of significant cracks. In this way the collapse mechanism does not de-pends by the materials’ limit strength, but it is due to the incapability of the structure to fit the horizontal and vertical displacements of the abutments. As a consequence it is clear that similar displacements should be considered when strengthening masonry arches introducing only systems, which are able to realize an effective “reinforcement” without chang-ing the constructive features of the structure.

Conversely, existing methods of repair often have been based on the idea of working over to make the structure resistant to the seismic actions, changing, in this way, the main features of global behavior.

According to this, it has to avoid, for instance, that methods (i.e., concrete slab), which, in order to limit the effects produced by the thrusts, try to elimi-nate them, changing, as a result, the static behavior of the structure, that is so reduced to a simple “ceil-ings”. For the same reasons, the use of removing the spandrel fill should be assess with care, taking into account the geometric configuration and the crack-ing pattern of the vault.

Such problems have led researchers to suggest strengthening masonry vaults with fiber – reinforced polymer (FRP) composites in the form of bonded surface reinforcements. There are several advantages related to this strengthening technique: very low weight, corrosion immunity, high tensile strength and low thermal expansion coefficient. Moreover the somewhat easiness of execution of the intervention, even in difficult operative conditions, allows a wide range of possible applications in several situations of damage, without considering that the possibility of binding or wrapping structural elements made of brittle materials (like masonry) allows, in most cases, to avoid the collapse of the structure and so assure the pursued safety conditions. Nevertheless, their lack of fire resistance and their relatively high cost may represent an obstacle for a widespread use.

According to these difficulties, beside the “tradi-tional” FRP, the use of a new family of composite materials, based on high strength twisted steel wires embedded within a cementitious grout (Steel Rein-forced Grout), is more and more considered. The core of the project is based on the idea to combine, along with the traditional advantages of the compos-ite materials, the performances of a material that, us-ing twisted steel wires, allows the same applications of FRP materials with lesser costs. Without consid-ering that the use of SRG, because of the presence of a cementitious grout, which replaced the polymeric resin of FRP materials, allows to increase the fire re-sistance as well.

The flexural limited resistance of the masonry vaults or arches can be overcome by introducing “passive” reinforcing steel strip (Borri et al. 2007a), making it able to sustain substantial bending mo-

ment in addition to axial loads, but a better applica-tion of the method is the use of pre-stressed strips. Loading the vault in radial direction, the SRG rein-forcement increase its compression and improve its resistance to pressure-flexure induced by incidental loads.

The consolidation effect is realised by simply placing one or more strips alongside the extrados surface of the vault. The strips are fixed to the ma-sonry of the supporting walls and then pre-tensioned (Figure 1). This fact implies the transmission of ra-dial self-equilibrated forces between the curved strips and the arch.

Figure 1. Prestressing device (courtesy Eng. Giannantoni).

As the compression resistance of the masonry is

usually high, it is possible, and not risky, to strongly rise the axial internal load in the masonry, avoiding the formation of the four-hinges collapse mecha-nism. All the structural section of the masonry will be more compressed as in the original state, thus postponing the formation of the cracks. Using this technique, the reinforcement does not interfere with the in situ material and respect the structural behav-iour of the existing building.

The method permits a recognisable sign of con-temporary interventions. The reinforcement so ap-plied works as an “active system”, which allows calibrating the actions as it needs and, if loss of pre-stress takes places, allows re-tensioning.

Even in case of variation of curvature along the arch ring, the reinforcement strips act in a beneficial way, as the mutual forces applied by the cords per-pendicularly to the surface are maximal exactly in the zones where the radius of curvature is minimal. It has to be noticed that the proposed consolidation technique works well only if the piers are able to sustain the lateral thrust induced by the arch (or vault). If they were too weak, the structure would break in a section somewhere between the springing and the keystone. This means that, in this case, the reinforcement have to be prolonged till the base of the piers. They have to help balancing load across the spans and thus they have to be placed up to the spans. In some case shear failure may take place in the structure and a sliding failure mechanism can occur. The reinforcement, although less efficient

than in the flexural induced collapse case, still bring to an increase of ultimate load.

3 CASE OF STUDY: CLOISTER PORTICO

3.1 Investigation and field survey The focused structure is a cloister portico, dated from the late 18th century, affected by the Umbria-Marche earthquake (1997). Such portico is consti-tuted by seven masonry arches and is built on the el-liptical plan of an old Roman amphitheatre (II B.C.) in the city of Spoleto (Italy).

All arches have a semi-circular shape and are built with natural stone bricks arranged in two layers bonded together by mortar joints only. The bottom layer, 155 mm thick, is built with 70x155x310 mm bricks laid on edge, whereas the upper layer, 35 mm thick, consists of 35x155x310 mm bricks laid flat.

All arches have exactly the same span length (3000 mm) and depth (480 mm), with the exception of the fourth arch that has a span of 3310 mm.

The arch piers are also made of natural stone bricks (70x155x310 mm) and are 480x480 mm square. Each pier includes a Doric capital, 270 mm high, whereas only the first three columns have a base (210 mm high).

Figure 2. Cloister Portico: plan view.

At the current state of the design process, the ret-

rofitting intervention involves only one of the seven arches of the cloister portico (label 04 of Figure 2).

Table 1. Geometric properties of ARCH 04. ARCH 04 Span (mm) 3310 Rise (mm) 1660 Section (mm x mm) 200x480 Height of the left pier (mm) 1800 Height of the right pier (mm) 1780

3.2 Retrofitting design To increase the structural seismic strength and, con-sequently, the safety of the structure, SRG strips, pretensioned through an innovative prestressing de-vice, has been designed. Such strips will be reused in the final intervention, when a new floor will be

realized at the top of the abovementioned cloister portico. The consolidation effect is realized by sim-ply placing two SRG sheets alongside the extrados surface of the arch. The sheets are fixed to the ma-sonry of the supporting piers and then tensioned. This fact implies the transmission of radial self-equilibrated forces between the curved sheets and the arch. The masonry of the arch will be conse-quently compressed and the distinct blocks will be helped to better support flexion, especially origi-nated by asymmetrical conditions. Actually, the de-signed prestressing stress is quite low (5% of SRG strip ultimate strength) because the main goal is to obtain a reinforcement able to work as an “active system”, so that composites start working even for low intensity seismic activity.

The prestressing device, used in such applica-tions, consists of three regions, comprising of two anchoring and one loading regions, respectively (Figure 3). The anchorage regions consists of: re-movable steel plates, to anchor the SRG sheets, and a fixed steel plate, anchored to the arch abutment, to hold the removable steel plates and bonded sheets through steel bolts and nuts. The loading regions consist of: two inequal angles and a winding axis, to prestress the SRG sheets manually with a dynamom-etric wrench, and two fixed and as many removable plates to bond sheets through steel bolts and nuts.

ANCHORAGE REGION

SRG STRIP

LOADING REGIONFIXED PLATE + REMOVABLE PLATES

ANCHORAGE REGIONFIXED PLATE + REMOVABLE PLATES

INEQUAL ANGLES + WINDING AXIS + ANCHORS PLATE

Figure 3. Layout of the intervention.

The strips used are 100 mm in width and 0.89

mm in thickness. Manufacturer’s mechanical proper-ties (Hardwire 2002) of the strengthening material are reported in Table 2.

Table 2. Properties of the laminate. 3SX-12 Tensile load (N/mm) 635 Elastic modulus (N/mm2) 210000 Ultimate strain (%) 1.2

A deflectometer has been used to register deflec-

tions. Thus it has been possible to monitor the arch crown deflection during and after the intervention.

Figure 4. Deflectometer used to register arch crown deflection.

3.3 Installation In order to ensure the safety of the structure, the arch rehabilitation started with the installation of scaf-folding on the intrados both of the above mentioned arch and of the two adjacent arches. This action is necessary to supply, after the removal of the filling material, the horizontal thrust of such arches and to bear tensile stresses generated by the prestressing of the SRG sheets.

After surface cleaning by sanding and water based solvents and then levelling the surface of the outer arch area, bedding bands were created using suitable cementitious grout.

The first step in the assemblage of the prestress-ing device consist of fastening the anchoring steel plates to the arch abutment through the use of two Φ 16 anchoring rods, inserted vertically, long enough (1000 mm) to reach the height of the pier. Each an-choring plate consist of three parts: a central plate (440×300×6 mm) with two round holes for the an-choring rods and two lateral plates (440×150×6 mm) joined to the central plate by a butt hinge. Anchor-age of the SRG sheets is created by bonding the ends of the sheets on the aforesaid lateral plates with a polymeric resin and by fastening two removable steel plates (440×150×6 mm) through the use of steel bolts. The removable plates are fixed to the lat-eral plates by the tightening of the nuts in the an-chorage region. High pressure must be applied to the SRG sheets through steel plates and ten bolts at each end to prevent slipping of the SRG sheets, which would result in a loss of prestressing force. Test re-sults (Borri et al. 2007b) show that the friction re-sulting from the pressure was sufficient to anchor the sheets during prestressing.

Figure 5. Anchorage region.

The next step consist of creating the anchorage of the SRG sheets in the loading region.

At one end, anchorage of the SRG sheets is cre-ated by using the same anchoring steel plates as the anchorage region (Figure 6).

Figure 6. Bonding SRG sheets to the anchoring steel plate of the loading region.

Conversely, at the other end, the reinforcement is

fixed directly to the winding axis of the loading de-vice. Because of the dimensions of such element, which should not permit prestressing of the two sheets simultaneously, it becomes necessary to use an intermediate steel plate (440×150×6 mm), placed at a position 250 mm distant from the loading de-vice. Using such a steel plate (Figure 7), it is possi-ble to fasten the two SRG sheets, fixed to the arch abutment, to the SRG sheet, fixed to the loading de-vice. As in the previous cases, the anchorage is cre-ated by fixing a removable steel plate (440×150×6

mm) to the intermediate steel plate through the use of steel bolts.

Figure 7. Fixing SRG sheets to the loading device.

After the anchorage was created, the desired

prestressing stress can be smoothly achieved in the SRG sheets by tightening the winding axis manually with a dynamometric wrench. As said, since the main goal is to obtain a reinforcement able to work as an “active system” and not to increase arch com-pression, the sheet was prestressed only up to 5% of its ultimate strength (3000 N). After the reinforce-ment was prestressed and fixed to the anchoring plate through the use of a removable plate, the sheet was cut and the loading device was removed.

Test results showed that the mechanical device proved to be practical and safe for prestressing SRG sheets. In particular the deflectometer used during the intervention has not register deflection, confirm-ing that the prestress load has been very low.

Figure 8 shows the details of the anchoring and loading regions after the assemblage is fully com-pleted.

Figure 8. Arch after intervention.

4 CASE OF STUDY: JACOBILLI BUILDING

4.1 Investigation and field survey Jacobilli building is a clustered complex that takes almost half of a single-standing block in the histori-cal centre of Foligno (Italy). The building, that in-cludes various different structural nuclei affected by changes and modifications during centuries, gained almost stable configuration around the XVIII cen-tury as a noble house.

The building was seriously damaged by the Umbria-Marche earthquake (1997) and may of its structures were repaired including some composite strengthenings. The present paper deals, in particu-lar, with the design process of the retrofitting inter-vention of two masonry vaults of the building. Both vaults are located in the building’s first level and, more in detail, in the Music and Sacrifice room.

Figure 9. First floor: longitudinal section and plan view (cour-tesy Eng. Menestò).

Both vaults are cloister vaults built with solid

clay bricks arranged in a single layer. Based on the survey, it was determined that the length of the vault

in the Music room is 7.90 m, width is 6.75 m, while its average thickness is 120 mm. Conversely, as for the vault in the Sacrifice room, the length is 7.14 m, width is 4.74 m and its average thickness is 120 mm.

SN

W

E

CRACKS a)

SN

W

E

CRACKSLOSS OF CURVATURE b)

Figure 10. Cracking pattern: a) Sacrifice room; b) Music room. In both cases, the analysis of the cracking pattern

reveals an asymmetric distribution of the cracks (Figure 10). The main cracks are in fact concen-trated along the façade wall (west side of the Sacri-fice room and east side of the Music room). Also, both vaults are affected by large cracks distributed along the ribs. According to this, it would seem rea-sonable to assume that the cause of such a collapse mechanism is due to the incapability of the façade wall to supply the horizontal thrust of the vault.

Also, at the extrados of the vault in the Music room a significant loss of curvature near the façade wall has been found during filling material removal. It is very likely that such permanent deformation was caused by the changes and modifications that affected the building during centuries. In particular, it can be noted that from 1712 to 1724 Giuseppe Jacobilli enlarged the mansion reaching wath is to-day Via Antonietti, incorporating other buildings in the process. As a consequence, the peripheral ma-sonry facing of the Music room and a portion of the vault were demolished and rebuilt (Figure 11). It seems reasonable to assume that such an interven-tion modified the static behaviour of the vault, re-ducing the curvature and therefore the bearing ca-pacity.

FACADE WALLDEMOLISHED IN 1712

GROINDEMOLISHED IN 1712

VAULTBUILT IN 1670

GROINREBUILT IN 1713

FACADE WALLREBUILT IN 1713

Figure 11. Enlargement of the Music room (1712 – 1713).

4.2 Retrofitting design Both vaults were reinforced with SRG tapes at the extrados intended to lock-out some of the most prob-able failure mechanisms. As above mentioned, the collapse mechanism does not depends by the materi-als’ limit strength, but it is due to the incapability of the façade wall to supply the horizontal thrust of the vault. As a consequence, it is clear that such a thrust should be considered introducing only systems, which are able to realize an effective “reinforce-ment” without changing the constructive features of the vault.

A traditional solution could be the substitution of filling material with hollow brick walls, which has positive effects thanks to the dead load decrease.

Conversely, as the reinforcement can bear the stresses occurring at the tensed edges, the applica-tion of composite laminates, as externally bonded strengthening materials, can modify the failure mode of the masonry vault and significantly increase the load – carrying capacity (Figure 12a). Therefore, the brittle failure of such structures, typically caused by the formation of four (or three) hinges, can be avoided. Depending on the position of the laminate, in fact, the formation of the forth hinge can be pre-vented (Foraboschi 2004). When the connection be-tween vault abutment and reinforcement is effective, their use also prevents the formation of cylindrical hinges in the piers and it causes high increases of in-ducing mechanism activation loads (Figure 12b).

Figure 12. Collapse mechanism: a) extrados reinforcement; b) extrados reinforcement + anchoring.

The consolidation effect is realized by simply

placing a reticular system, made of transversal and

ba

longitudinal prestressed SRG strips on the extrados of the vaults. Where the transversal strips are used to resist the horizontal thrust acting on the façade wall, whereas the longitudinal strips are used as a connec-tion element between the transversal strips.

SN

W

E

TRANSVERSAL STRIPS

TRANSVERSAL STRIPS

LONGITUDINAL STRIPSLOADING REGION

ANCHORING REGION

ANCHORING REGION

ANCHORING REGION

LOADING REGION

ANCHORING REGION

ANCHORING REGION

LOADING REGIONANCHORING REGION

SN

W

E

LOADING REGION

ANCHORING REGION

ANCHORING REGION

ANCHORING REGION

TRANSVERSAL STRIP

LONGITUDINAL STRIP

Figure 13. Layout of the interventions.

As in the previous case of study, by using the same prestressing device, the sheets are fixed to the masonry of the supporting piers and then tensioned.

The strips used are 100 mm in width and 0.89 mm in thickness. Manufacturer’s mechanical proper-ties of the strengthening material are reported in Table 2. Even in this case, a series of deflectometers have been used to register deflections. Thus it has been possible to monitor the vaults deflection during and after the intervention.

4.3 Installation The rehabilitation of the two vaults started with re-moval of the filling material up to the haunches, where the solid clay bricks of the structure are in-serted into the outer wall. In both cases, at the vault extrados a horizontal wooden ring have been found during filling material removal. This ring served to provide structural support for the vault thrust.

After surface cleaning by sanding and water based solvents and then levelling the surface of the outer vault area, bedding bands were created using suitable cementitious grout. It should be noted that,

despite careful preparation, areas with abrupt varia-tions in curvature may occur. In these cases experi-mental tests showed high degree of weakness of “traditional” FRP sheets. Conversely, because of their higher shear strength, the use of steel fibers may overcome such shortcomings.

Even in this cases, the first step in the assemblage of the prestressing device consist of fastening the anchoring steel plates (220×150×6) to the vault abutment through the use of two Φ 16 anchoring rods inserted diagonally, long enough to reach the height of the springer. High pressure has been ap-plied to the SRG sheets through steel plates and three bolts at each end to prevent slipping of the sheets.

Figure 14. Anchoring of the SRG sheets.

After the SRG sheets have been fixed, the an-

chorage of the sheets in the loading region was cre-ated. More in detail, at one end the sheet was an-chored through the use of the same steel plates as the anchorage region; whereas at the other end, the sheet was fixed directly to the winding axis of the loading device.

Figure 15. Loading region.

Finally, the desired prestressing stress has been

achieved in the SRG sheets by tightening the wind-ing axis manually with a dynamometric wrench. Even in this cases the prestressing load was quite low. The sheets were prestressed only up to 6% of their ultimate strength (4000 N). After the rein-forcement was prestressed and fixed to the anchor-ing plate through the use of a removable plate, the sheet was cut and the loading device was removed.

Figure 16. Prestressing of the SRG sheets.

Also, it can be noticed that prestressing SRG

strips permits the reinforcement to follow a regular curve parallel to the original ideal surface of the un-deformed vault. According to this, in the Music room, connected with the loss of curvature (near the façade wall) in addiction to the reinforcement, steel flat plates, bolted to the bricks, were used to secure the ply to the vault extrados.

This fact implies the transmission of radial self-equilibrated forces between the strips and the vault, allowing to reduce deformation and to re-stabilish, therefore, an adequate curvature.

T TF

SRG STRIP

STEEL FLAT PLATESVAULT

F F F F

FFFFFF

Figure 17. Stress state generated by the prestressing SRG strips.

Figure 18. Application of steel flat plates.

Figure 19 shows the details of the extrados of the

vaults after the assemblage is fully completed.

Figure 19. Vaults after intervention: Sacrifice and Music room.

Also, as it regards the Music room, because of the low value of the prestress load, the deflectometers used during the intervention have been register a re-duction of deformation of “only” 20% (11 mm) of the original value. It would seem reasonable to as-sume that an increase of the prestressing load should re-stabilish the original curvature.

5 CONCLUSIONS

The operations carried out, firstly to save and then to consolidate and restore the masonry arch of the cloister portico built inside a Roman amphithea-tre in the city of Spoleto (Italy), and of two masonry vaults of the Jacobilli building in the city of Foligno (Italy) have all followed the same philosophy. To place the most up-to-date techniques and technolo-

gies at the service of culture, in order to respect the historic value of the ancient buildings and to obtain adequate safety levels, whilst changing as little as possible the original structural conception. These technologies, never applied before in the field of res-toration, have been studied specifically for this occa-sion, offering new and interesting possibilities for the safeguard of the World architectural heritage.

6 REFERENCES

Borri, A., Casadei, P., Castori, G. & Ebaugh, S. 2007a. Ex-perimental analysis of masonry arches strengthened by in-novative composite laminates. Proceeding of the 10th North American Masonry Conference (10NAMC), St. Louis, Missouri, USA, June 3-6, 2007.

Borri, A., Castori, G., Giannantoni, A. & Grazini, A. 2007b. Performance of reinforced masonry bond beams. Proceed-ing of the 10th North American Masonry Conference (10NAMC), St. Louis, Missouri, USA, June 3-6, 2007.

Foraboschi, P. 2004. Strengthening of masonry arches with fi-ber-reinforced polymer strips. Journal of Composites for Constructions, ASCE 8(3), 2004, pp 7-16.

Hardwire llc., 2002. What is Hardwire. Product Guide Specifi-cation. Web site: http://www.hardwirellc.com.

Huang, X., Birman, V., Nanni, A. & Tunis, G. 2005. Properties and potential for application of steel reinforced polymer and steel reinforced grout composites. Composites, Part B, Vol. 36, 2005, pp 73-82.

Jurina, L. 1997. The reinforced arch: a new technique for strengthening masonry arches and vaults using metal tie bars. Proceedings of 16th National Congress of C.T.A., An-cona, Italy ,1997.

Matana, M., Galecki, G., Maerz, N. & Nanni, A. 2005. Con-crete substrate preparation and characterization prior to ad-hesion of externally bonded reinforcement. Proceedings of International Symposium on Bond Behaviour of FRP in Structures (BBFS 2005), Hong Kong, China, 2005.

Triantafillou, T.C. 1998. Strengthening of masonry structures using epoxy-bonded FRP laminates. Journal of Composites for Constructions, ASCE 2(2), 1998, pp 96-104.

Valluzzi, M.R., Valdemarca, M. & Modena, C. 2001. Behav-iour of brick masonry vaults strengthened by FRP lami-nates. Journal of Composites for Constructions, ASCE 5(3), 2001, pp 163-169.